The present invention relates to an optical element for enlarging an image displayed on an image display element.
In general, to obtain a good optical performance in an optical device using an optical element such as a prism, or the like, it is necessary to position the optical element with respect to a cabinet and to other optical element and to fix it in the cabinet or to the other optical element accurately with a pinpoint accuracy.
The optical element is typically represented by an optical prism. Since the optical prism has the smaller number of flat surfaces acting as a reference, there are conventionally proposed various methods as a fixing method of it.
For example, an optical prism 1 as a prior example shown in FIG. 1 is arranged such that flange portions 1a project from two confronting surfaces that do not contribute to the optical action of the optical prism 1, screw insertion holes 1b are drilled through the flange portions 1a as well as positioning pins 1c project from the back surfaces of the flange portions 1a. 
In contrast, a cabinet 2 to which the optical prism 1 is fixed has joint surfaces 2a against which the flange portions 1a provided with the optical prism 1 are abutted, and then screw holes 2b are threaded into the joint surfaces 2a at the positions thereof corresponding to the screw insertion holes 1b, and further positioning recessed portions 2c, into which the positioning pins 1c are inserted, are drilled into the joint surfaces 2a. 
In assembly, the optical prism 1 is fixed to the cabinet 2 by abutting the back surfaces of the flange portions 1a, which project from both the side surfaces of the optical prism 1, against the joint surfaces 2a formed on the cabinet 2, by positioning the optical prism 1 with respect to the cabinet 2 by inserting the positioning pins 1c, which project from the back surfaces of the flange portions 1a, into the positioning recessed portions 2c drilled into the joint-surfaces 2a, and by driving screws 3, which have been inserted through the screw insertion holes 1b, into the screw holes 2b drilled into the joint surfaces 2a. 
In this case, as shown in FIGS. 2 and 3, each flange portion 1a may be cut to shorten its size up to the position where the screw insertion hole 1b is halved, whereas a projecting portion 2d having a height slightly lower than the thickness of the flange portion 1a may be projected from the outside surface of each joint surface 2a of the cabinet 2.
That is, in the arrangement shown in FIGS. 2 and 3, when the flange portions 1a projecting from both the sides of the optical prism 1 are inserted between the projecting portions 2d of the cabinet 2, circular screw insertion holes are formed by the halved screw insertion holes 1b drilled through the flange portions 1a and halved screw insertion holes 2e formed on the inside surfaces of the projecting portions 2d, and the screws 3 having been inserted through the screw insertion holes are driven into the screw holes 2b drilled into the joint surfaces 2a of the cabinet 2, thereby the optical prism 1 is fixed to the cabinet 2.
As described above, since the flange portions 1a are formed in the halved state in FIGS. 2 and 3, it is possible to reduce the shape of the flange portions 1a as compared with the Free-Form-Surface prism 1 shown in FIG. 1. Further, when the screws 3 are tightened, the heads thereof are abutted against the projecting portions 2d on the cabinet 2 side and the screws 3 are prevented from being more tightened, thereby the occurrence of internal stress to the optical prism 1 is suppressed so that the deformation of the optical prism 1 can be prevented.
Further, as shown in FIG. 4, there is also known a technology for fixing the optical prism 1 to the cabinet 2 by drilling screw holes 2f through the side surfaces of the cabinet 2 and by pressing the side surfaces of the optical prism 1 by the extreme ends of set screws 3b driven into the screw holes 2f. 
According to this prior example, the shape of the flange portions 1a can be more reduced because it is not necessary to drill insertion holes, through which tightening screws are inserted, through the flange portions 1a of the optical prism 1. As a result, the reduction in size of the cabinet 2 for holding it can be realized, and thus a device can be reduced in size in its entirety.
In this case, as shown in FIG. 5, the extreme ends of the set screws 3b do not directly press the side surfaces of the optical prism 1 by attaching a sheet member 4 bent in a U-shape to the inner surface of the cabinet 2 and by pressing the side surfaces of the optical prism 1 by the set screws 3b through the side surfaces 4a of the sheet member 4, thereby internal stress occurred to the optical prism 1 can be suppressed.
In contrast, as shown in FIG. 6, when the set screws 3b are arranged as pointed set screws 3c, V-shaped grooves 1d, into which the extreme ends of the pointed set screws 3c are inserted, are formed on the side surfaces of the optical prism 1, and the pointed set screws 3c are driven into the screw holes 2f threaded into the cabinet 2, the extreme ends of the pointed set screws 3c impinge on the slants of the V-shaped grooves 1d formed in the side surfaces of the optical prism 1.
When the pointed set screws 3c are further driven, the optical prism 1 is pulled in an impinging surface direction and pressed as well as can be fixed to the cabinet 2. Note that reference numeral 2g denotes pins that are inserted into guide holes drilled through the flange portions 1a and regulate the movement of the optical prism 1 in a width direction.
Further, as shown in FIG. 7, there is also known a technology for interposing elastic fixing members 5 such as spring between both the side surfaces of the optical prism 1 and the inner surfaces of the cabinet 2.
According to this prior example, a fixing and pressing force is made constant to the optical prism 1. Further, when the optical prism 1 is expanded or contracted by a temperature change, or the like, the deformation of the optical prism 1 is allowed by the elastic deformation of the elastic fixing members 5, thereby the occurrence of internal stress is prevented, thereby the breakage of the optical prism 1 can be prevented before it occurs.
Further, an optical prism 11 as shown in FIG. 8 is also known. This optical prism 11 is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 9-73005. The optical prism 11 shown in FIG. 8 is used in an image observation device such as a head mount display (HMD), or the like for displaying an image displayed on a small image display element in enlargement. The optical prism 11 has a first light incident optical surface 11a composed of a flat surface or a curved surface, a second optical surface 11b composed of a flat surface or a curved surface for totally reflecting light beams incident in an element from the first optical surface 11a, a third optical surface 11c for reflecting at least a part of the light beams from the second optical surface 11b to the second optical surface 11b side, and optically positioning flange portions lid disposed to two side surfaces, which confront each other and do not execute an optical action, other than the first to third optical surfaces 11a-11c. 
There is disclosed a technology for accurately holding the optical prism 11 without causing optical distortion by holding the optical prism 11 through the flange portions 11d. 
However, the conventional examples described above have the following problems.
In the optical prisms 1 and 11 shown in FIGS. 1 and 8, the flange portions 1a and 11d project from the side surfaces thereof, which increases the sizes of the optical prisms in y-directions shown in FIGS. 1 and 8 and acts as an obstruction when miniaturization of the overall device is attempted.
Further, in the optical prism 1 shown in FIGS. 2 and 3, an unnatural force is prevented from acting on the optical prism 1 because a tightening force is regulated to some extent by the projecting portions 2d formed to the cabinet 2 when the screws are tightened. However, it is difficult to perfectly prevent the action of the unnatural force. As a result, strain is caused to the optical prism 1 when the dimensions of respective components are erroneously determined, the respective components are erroneously set, and the assembly conditions thereof are erroneously set, from which a disadvantage arises in that the deterioration of an optical performance is hastened.
In contrast, the respective optical prisms 1 shown in FIGS. 4-6 are fixed to the cabinet 2 by directly pressing the side surfaces of the optical prisms 1 using the set screws 3b or the like. Thus, a disadvantage arises in that internal stress is liable to be caused to the optical prisms 1 by the tightening force of the set screws 3b or the like and the optical performance is deteriorated.
Further, since the expansion rate of the optical prism 1 is different from that of the cabinet 2, there is a disadvantage in that strain is liable to be caused to the optical prism 1 when it is subjected to the influence of a temperature change and the optical performance is deteriorated thereby.
Further, in the optical prism 1 shown in FIG. 7, the side surfaces of the optical prism 1 are fixed to the inner surfaces of the cabinet by the elasticity of the elastic fixing members 5, the influence caused by the difference between the expansion rate of the optical prism 1 and that of the cabinet 2 can be absorbed by the elastic deformation of the elastic fixing members 5, and thus the optical prism 1 is unlike to be subjected to the influence of the temperature change. However, there is a disadvantage that when an external force such as vibration, impact or the like is applied to the optical prism 1, the elastic fixing members 5 are plastically deformed and the position of the optical prism 1 is liable to be dislocated thereby.
An object of the present invention is to provide an optical element and an image observation device capable of realizing the miniaturization of the device in its entirety by effectively utilizing a space between the optical element and a cabinet and moreover capable of fixing the optical element to the cabinet without the occurrence of optically harmful strain.
Further, another object of the present invention is to provide an image observation device capable of realizing the cost reduction of a product by decreasing the portions of an optical element which require a dimensional accuracy as well as capable of being relatively easily assembled without influencing optical characteristics and capable of realizing the miniaturization of the product.
An optical element for enlarging an image displayed on a small image display element is characterized by including a first optical surface for capturing the image approximately facing the display surface of the image display element, a second optical surface for reflecting the light beams incident from the first optical surface in the inside of the element as well as for finally causing the light beams of an enlarged image to proceed to the eye balls of an observer, at least one reflection optical surface for contributing to at least one instance of internal reflection so that the light beams incident from the first optical surface reach the second optical surface, and side surfaces that do not contribute to optical action, wherein element fixing mounting portions are disposed to any of the first to second optical surfaces and the reflection optical surface.
Further, an optical element of the present invention for enlarging an image displayed on a small image display element is characterized by including a first optical surface for capturing the image approximately facing the display surface of the image display element, a second optical surface for reflecting the light beams incident from the first optical surface in the inside of the element as well as for finally causing the light beams of an enlarged image to proceed to the eye balls of an observer, at least one reflection optical surface for contributing to at least one instance of internal reflection so that the light beams incident from the first optical surface reach the second optical surface, and side surfaces that do not contribute to an optical action as well as including element fixing mounting portions, wherein the element fixing mounting portions are formed so as to be separated from the optical element by grooves formed on at least any two surfaces of the respective surfaces by which the optical element is formed.